The present invention relates to a mobile communication system, in particular, relates to structure of cells in such a communication system.
FIG. 1 shows cells in a mobile communication system, in which the same channels are re-used with some spacing. The thick line in FIG. 1 shows a unit of a cluster. Each hexagon shows a cell, and a numeral in a cell shows a channel. The same numerals show the same channels which use the same frequency. The same channel or the same frequency is re-used in a cell in another cluster. The structure of a cell may be either an omni-cell as shown in FIG. 2(a) in which a base station having a non-directional antenna is located around a center of a cell, or a sector cell as shown in FIG. 2(b) in which a directional antenna divides an area into a plurality of sectored cells each of which is assigned a specific channel or frequency.
A re-use partition considers a virtual small cell, which uses the same frequency with the shorter spacing than that of cells. The re-use technique increases the spectrum efficiency in a mobile communication system.
An idea of a re-use partition is described in accordance with FIG. 3. FIG. 3(a) shows an example of the use of the same frequency. A circled line shows a cell, a symbol (+) shows a base station around a center of a cell, a FIG. (1,2,3) shows a channel number, and d shows the spacing of the use of the same frequency. The same FIG. (1,2 or 3) shows the same channel or the same frequency. In the example of FIG. 3(a), the spacing of the same channels is three cells (d=3).
FIG. 3(b) and FIG. 3(c) show a typical re-use partition. FIG. 3(b) shows the spacing of outer cells, and FIG. 3(c) shows the spacing of inner cells, when virtual cells are introduced to the cell layout in FIG. 3(a). As shown in FIG. 3(b), the spacing of outer cells is three cells (d=3) which is the same as that of FIG. 3(a), on the other hand, the spacing of inner cells is two cells (d=2) in the example of FIG. 3(c). Since the spacing or the spectrum efficiency of inner cells is improved by a re-use partition, the total efficiency of the use of frequencies is improved. The spacing or the efficiency of inner cells improved by improving a cell selection accuracy to determine which cell (outer cell or inner cell) a mobile station is located in.
In a prior re-use partition system, a control channel for mobile communication for connection et al is common to both an outer cell and an inner cell. In other words, both an outer cell and an inner cell are handled as a single control cell. As shown in FIG. 4, a control channel for access is used commonly to outer cells and inner cells, and a speech channel has a specific transmiter for each outer cell and each inner cell.
In FIG. 4, an antenna is used commonly in outer channels, and inner channels. A plurality of tranceivers (TRX.sub.out) for outer cells, and a plurality of tranceivers (TRX.sub.in) for inner cells, and a single common controller channel tranceiver (TRX) are coupled with the antenna through an antenna multiplexer. Those tranceivers are coupled with the base station control, and the communication channels (external channels).
Accordingly, a prior re-use partition system in a mobile communication system has the disadvantages as follows:
First, a mobile station must be located to determine whether it is either in an inner cell or an outer cell in every access and during speech. If the decision is effected based upon a received level at a base station, operational load of a base station increases since the decision must be effected in a short time in connection time, and/or the decision must be always carried out during speech.
Further, if the decision is carried out merely based upon whether a received level exceeds a predetermined threshold level or not, the decision would not be accurate due to fading which is specific to mobile communication. In that case, it would be determined incorrectly as if a mobile station located in an inner cell merely because the received level is high, while the mobile station is located in an outer cell, and would increase undesirable interference.